In the past three years, we have seen a significant increase in the number of dogs where heartworm preventive medication has not been 100% effective. In addition, it has been more difficult to clear infected dogs of the worm burden with treatment. It has also been more difficult to clear microfilariae from dogs post treatment. Some dogs who have been treated and cleared have heartworms again the next year, despite year-round preventive medicine. These reports come from the Mississippi valley, starting about 100 miles south of St.Louis, and getting worse as one goes south.
For my own part, it was as though in the year 2006 "the rules changed". Many veterinarians and dog-owners have become convinced that a resistant population of heartworms has developed. With no new drugs in the pipeline, this is a major concern.
As late as October of 2008, educational meetings, publications and discussions tended to dismiss the problem as merely one of perception, rather than an actual change in the incidence of the disease.
In April of 2009, I was privileged to be one of six veterinarians from private practices invited to a round-table discussion seminar. Our input was sought in regard to our experience with heartworm disease, its prevention and treatment, and our problems with all of these.
Three leading parasitologists, all involved with heartworm research, opened the discussion by saying, “We know that something has changed, but we don’t know what it is. There is a problem, but the underlying cause has not been determined. We are here to gather and share information.”
This was a refreshing change. You know what they say: the first step is admitting you have a problem.
Everyone now concedes that something has been different in the past three years. There was a sudden “uptick” to a higher level of problem (observed in my practice in 2006 and continuing over 2007 and 2008), rather than a slowly growing problem. In other words, we went from “Plateau A” to “Plateau F” without going through B,C,D,&E. The change seemed abrupt, rather than gradual. This speaks against the development of a resistant population of heartworms. There are other epidemiological objections to this, as well.
The parasite has a very long life cycle: six months minimum. It requires a vector, the mosquito. Reservoirs are widespread and act as carriers for years, and more than half of the reservoir hosts are never treated. Coyotes are a big reservoir of the disease, but their rate of infection has not been studied anywhere except in California. There are, of course, also many, many dogs in communities and the countryside that do not receive preventive medicine, and act as reservoirs of the disease.
This makes the development of a resistant population of worms by “Darwinian” selection almost impossible.
On the other hand, as yet no other reasonable explanation has been put forward. An increase in the numbers of mosquitoes might explain it, but there is no data to support or refute this. There just is no helpful mosquito data. Our subjective experience locally is that the mosquitoes are terrible, but they have been so for many years.